The invention relates to a method for producing a metal strip, in which the strip is rolled in a multi-stand rolling mill, is output after the final rolling stand of the rolling mill in the direction of conveyance and is cooled in a cooling device, wherein, immediately after passing the working rolls of the final rolling stand, the strip or plate is subjected to an additional rapid cooling, the cooling of the strip or plate at least partially taking place still within the extent of the final rolling stand in the direction of conveyance, the rapid cooling taking place by a cooling medium being applied to the strip or plate from above and from below.
A method of the type in question is known from US 2012/068391 A1 and from JP S60 243226 A. WO 02/070157 A1 and JP S60 221115 A show other solutions.
The mechanical properties of steel materials can be influenced in various ways. An increase in the strength is achieved by adding certain alloying elements (solid-solution hardening). Furthermore, during the rolling the finishing train temperature may be lowered, in order to achieve a higher dislocation density (dislocation hardening). By alloying with microalloying elements—such as for example Nb, V or Ti—precipitations are formed, causing an increase in the strength (precipitation hardening). However, these mechanisms have the disadvantage that the toughness is adversely influenced. By contrast, a fine grain structure of the microstructure (fine-grain hardening) has a positive effect on the strength properties and at the same time the toughness properties. With a small grain size, the strength and toughness properties of the steel material are improved.
It is typically provided in hot-strip or plate-rolling trains that there is a distance of 12 to 20 m between the final rolling stand and the cooling zone. Generally, measuring devices for the temperature, thickness, profile and flatness are installed in this region. Especially in the case of slowly rolled strips, the time until cooling is achieved can consequently be 12 to 20 seconds (at a strip speed of 1 m/s). However, this has a disadvantageous effect on the grain size of the microstructure within the strip, and consequently on the achievable mechanical properties, since recrystallization and recovery processes occur after the forming process.
It is disadvantageous that after the rolling of the strip or plate there is a pronounced increase in the grain size in the microstructure, with concomitant recrystallization and recovery processes. The increase in grain size leads to a deterioration in the mechanical properties.
A further aspect concerns the flatness of the strip or plate. The lower the temperature after the cooling in the cooling zone and the thicker the strip or plate thickness, the more important the amounts of water applied to the upper side and underside of the strip. If the ratio of the amounts of water between the upper side and the underside is not optimum, the strip or plate becomes unflat or uneven. In this case, laborious reworking or rectification is required.